This invention relates to packing elements for exchange columns, particularly heat exchange columns, intended more particularly for countercurrent exchanges by putting a gas current and solid particles flowing by gravity in direct contact within the packing.
It relates very particularly to the processes and devices described in U.S. Pat. Nos. 4,423,558 and 4,450,895 which resort to the average speeds of gas currents within the packing, preferably on the order of the maximum free fall speed of the particles.
Actually it has been found that packing elements currently used for exchanges between gases and liquids of the Pall ring type, obtained by the operations of cutting plates, provided with various notches, from steel sheet, then by shaping operations generally leading to cylinders provided with lugs on the inside, do not always obtain totally satisfactory results for gas-solid exchangers of the type described in the patents cited above. In the presence of particles with mediocre flow characteristics, particularly because of their shape and/or high granulometries, use of these elements actually, particularly for high speeds aimed according to the teachings of U.S. Pat. No. 4,423,558 at optimizing the efficiency of the exchanger, makes it possible to observe a trapping of the particles by the packing elements, leading to an increase in the retention of particles within the packing, and consequently to an increase in the pressure drop of the gas current through the packing and especially to a segregation of the particles, or even complete clogging of the exchange column.
For example, when the Pall rings of heat-resistant steel sheet, 25 mm in diameter and 25 mm high, are placed loose on a support consisting of a large-mesh (60.times.20 mm) grill formed with heat-resistant steel strips 15.times.1 mm, set edgewise and spot welded, particles, for example, silica or zirconia sands, even fairly spherical, with average granulometries greater than 2 mm, cannot be reliably treated under the conditions provided by the process of patent U.S. Pat. No. 4,423,558 because generally a complete clogging of the column is very quickly observed. Similar difficulties arise sooner or later, in industrial practice, when the product to be treated, even with an average granulometry much less than the critical value of 2 mm given above, contains some particles greater than 2 mm, either because they have escaped the preparatory grinding and screening operations or because they result from a process of uncontrolled agglomeration of the fine particles.
A certain improvement has been noted, especially in regard to the appearance of the phenomena of clogging the rings by particles, when these packing elements are placed on their support not loose but in an ordered manner, their axis of revolution being vertical; however, other drawbacks are then observed, particularly in regard to the efficiency of the treatment, as a result of spatial segregation, generally radial, between the particles and gas current, and a direct flow of the solid without slowdown.